The present invention relates to an improved semiconductor device which can be operated with less power than conventional semiconductor devices.
In the prior art, various semiconductor devices have been invented and the characteristics of semiconductor devices have been remarkably improved during the past ten years. An electronic wrist watch has been realized due to such a semiconductor device, and it is possible to operate such a watch for two or three years by the use of a little button type battery. However, social conditions require that energy consumption should be reduced to as little as possible to further increase battery life, and a semiconductor device having an operating current which is extremely small is intensely required. A complementary type insulated gate transistor semiconductor device (referred to as a CMOS-IC hereinafter) is known to have the lowest IC operating current. If the device operating frequency is given as f, the current I of the CMOS-IC will be described by the following equation: EQU I=CVf (1)
where, V is an applied voltage to the IC, and C is a capacitance of transistors used in the IC.
Therefore, the reduction of the current can be realized by selecting a small value of capacitance C, a low applied voltage V or a low operating frequency f. However, in the case of an electronic timepiece, the frequency of an oscillating circuit used as a time is almost completely determined by a resonance frequency of a quartz vibrator of the timepiece, which is selected in accordance with a required accuracy or a size of the timepiece. Recently, the frequency is almost always selected at 32 KHz. There is the possibility of the selection of a higher oscillating frequency, such as on the order of 4 MH.sub.3 by which greater timepiece accuracy can be realized, but it it not expected that a lower frequency will be selected.
On the other hand, the applied voltage V is determined in accordance with the type of battery that is used. Therefore, for the reduction of the current, there is no way other than the reduction of the capacitance C, and in order to decrease the capacitance C, the attempt of each semiconductor device manufacturer has been to increase the physical working accuracy of the device fabrication to make the size of the device transistors. Roughly speaking, it is said that the capacitance will be decreased to a quarter if the transistor dimensions decrease by a half. However, the present working size for an IC, in the case of the pattern for aluminum wiring, is 5 .mu.m, so that in order to realize a 2.5 .mu.m working size, semiconductor manufacturing equipment widely in use should be replaced with new equipment. Consequently, it follows that a great increase in cost of semiconductor devices would result.
FIG. 1 is a block diagram of the conventional IC for a timepiece. Circuit 2 is a quartz oscillating circuit of, for example, 32 KHz for a time base, 3 is a buffer circuit for shaping the output waveform from the oscillating circuit 2 and applying the shaped signal to a subsequent frequency dividing circuit 4. The frequency dividing circuit 4 receives a time signal from the buffer circuit 3 and divides the received signal so as to obtain a signal having a predetermined frequency such as 32 KHz. Element 5 is a driving circuit for a display device 6 in which the output from the frequency circuit 4 is changed into a time signal, and display device 6 is, for example, a liquid crystal display device for displaying a time in accordance with the signal from the driving circuit 5. To operate the conventional circuit, an AgO.sub.2 button type 1.5 volt battery 1 is usually used as a power source. The total amount of current I consumed in the circuit is the summation of the current I.sub.o in the oscillating circuit 2, the current I.sub.B in the buffer circuit 3, the current I.sub.D in the frequency dividing circuit 4 and the current I.sub.S in the driving circuit 5. EQU I.sub.T =I.sub.O +I.sub.B +I.sub.D +I.sub.S ( 2)
For example, in a typical digital quartz wrist watch, each current value is as follows: I.sub.O =0.5 .mu.A, I.sub.B =0.3 .mu.A, I.sub.D =0.2 .mu.A, and I.sub.S =0.5 .mu.A. Therefore, the total current I.sub.T will be as follows: EQU I.sub.T =1.5 .mu.A
A voltage step-up circuit for the display device 6 is omitted from FIG. 1. Furthermore, the current I.sub.s for the driving circuit 5 contains the current for the display device 6. In FIG. 2, some portions such as the oscillating circuit 2, the buffer circuit 3, etc. in FIG. 1, are illustrated in more detail. T.sub.1, T.sub.2, T.sub.3, T.sub.4, T.sub.5 and T.sub.6 are MOS transistors, 7 and 9 are capacitors for oscillation and 8 is a quartz vibrator.